1. Field of the Invention
This invention relates to nerve monitoring systems and to nerve muscle monitoring systems, and more particularly to nerve proximity and status detection methods and systems.
2. Description of Related Art
Systems and methods exist for monitoring a nerve. One such system determines when a needle is approaching a nerve. The system applies a current to the needle to evoke a muscular response. The muscular response is visually monitored, (typically as a shake or xe2x80x9ctwitchxe2x80x9d). When such a muscular response is observed by the user, the needle is considered to be near the nerve coupled to the responsive muscle. These systems require the user to observe the muscular response (to determine that the needle has approached the nerve). This may be difficult depending on the competing tasks of the user. In addition, when general anesthesia is used during a procedure, muscular response may be suppressed, limiting the ability of a user to detect the response.
Accordingly, a need exists for a better system and method that can determine the proximity of a needle, catheter, or other device to a nerve or group of nerves.
The present invention includes a method and system for determining the status of a nerve, in particular a nerve motor unit (where the nerve is connected to a set of muscle fibers which contracts). The present invention also includes a method and system for determining the proximity of a nerve (motor unit) to a conductive device. In order to determine the status of a nerve, the invention applies an electrical signal to a conductive device or element. The conductive element is positioned near the nerve. The electrical signal includes a number of current levels applied over time. Each of the current levels is applied to the conductive element and thus to the nerve. The response of the nerve to each current level is measured. The response of the nerve to the stimulus may be detected by an electromyogram (xe2x80x9cEMGxe2x80x9d). The EMG response of the nerve may be measured at a muscle that is physiologically coupled to the nerve (motor unit). In a preferred aspect, the present invention determines the status of the nerve based on the measured response of the nerve to the current levels applied thereto.
To determine the status of the nerve based on the response, the present invention may map the response of the nerve for each current level. The mapping is based on the level of each response at a fixed period of time or latency of the response. The present invention uses the mapping of the responses of the nerve to the current levels to determine the status of the nerve. This information may be used to determine the health of the nerve and may be gathered real time such as during a surgery or other medical procedure. In such a case, the conductive element may be coupled to a catheter, a needle, a cannula or another medical device. The fixed latency period used to map the responses may be determined as a function of peaks in each response. The maximum magnitude peak in each response is determined and then the latency period having the largest number of peaks located contemporaneously in the responses is selected as the fixed latency period to be used to map the responses. The sum of the ratio of peak level to current level for the maximum magnitude peak of each response may also be determined. The latency having the largest sum of ratios of peak level to current level is then selected as the fixed latency.
In order to determine the proximity of a nerve to a conductive element such as a proximity electrode, the present invention applies a first electrical signal to a calibration electrode. The calibration electrode includes a conductive element that is located or positioned near the nerve. The present invention then applies a second electrical signal to the proximity electrode. The first and second electrical signals include a number of current levels applied at different time intervals. Each of the current levels is applied to the proximity electrode and calibration electrode and thus to the nerve. The response of the nerve to each current level from each electrode is measured. For example, the response of the nerve to each stimulus may be detected by an electromyogram (xe2x80x9cEMGxe2x80x9d). The EMG response of the nerve may be measured at a muscle that is physiologically coupled the nerve. The present invention determines the relative proximity of the nerve to the proximity electrode by comparing the responses of the nerve to the first signal and to the second signal. Optionally, the present invention may also compare the slope (change in voltage/change to current (dV/dI)) of the responses of the nerve to the first signal and the second signal.
The present invention may also map the response of each current level of the first signal and map the response of each current level of the second signal. Then, the relative proximity of the nerve to the proximity electrode may be determined by comparing each mapping. The mapping of each signal is based on the level of response at a fixed period of time or latency of the response. The determined proximity may be used to guide a real time procedure such as during a surgery or other medical procedure. In such a case the proximity electrode may be coupled to a catheter, a needle, a cannula or another medical device. The fixed latency used to map the response of each signal may be determined as a function of peaks in each response. The maximum magnitude peak in each response is determined and then the latency having the largest number of maximum magnitude peaks located contemporaneously in each response is selected as the fixed latency. The sum of the ratio of peak level to current level for the maximum magnitude peak of each response may also be determined. The latency having the largest sum of ratios of peak level to current level is selected as the fixed latency accordingly.